Below is a list of the most common techniques used in base oil viscosity testing. Several methods have been established for viscosity testing, and each has its advantages. For the sake of comparison, absolute viscosity is often converted into kinematic viscosity by dividing it by the fluid’s specific gravity (SG).ĬSt = cP/SG or the equation’s inverse: cP = cSt x SG Kinematic viscosity is reported in centistokes (cSt), but absolute viscosity is reported in centipoise (cP). The key difference between the two is that kinematic viscosity is measured by observing the fluid’s resistance to flow under the force of gravity, while absolute viscosity is measured by observing the fluid’s resistance to flow under an external and controlled force, either through a capillary or by movement of a body through the fluid. Viscometers measure the viscosity of Newtonian fluids.Īmong the various viscometer methodologies, there are two distinct ways in which viscosity can be expressed: kinematic viscosity and absolute viscosity. The viscosity of these types of fluids is measured by rheometers. A fluid in which the viscosity changes with flow conditions is called a non-Newtonian fluid. As flow conditions vary, not all fluids retain an unchanged viscosity.
Viscosity is the measure of a fluid’s resistance to flow (shear stress) at a given temperature.
For this reason, it is important to understand not only the viscosity of the base oil but also how it may change due to fluctuating operating or environmental conditions. On the other hand, too high of a viscosity would force the machine to work harder to overcome the lubricant’s internal resistance to flow. Why is viscosity so important in the lubrication of a machine? For one, too low of a viscosity would allow interfacial contact between internal components in motion to generate wear. In tribology and the study of lubrication, viscosity has often been referred to as the most important characteristic of a base oil. Let's discuss the various styles of viscometers, their internal characteristics and factors that go into performing a precise and repeatable viscosity measurement. A viscometer is an instrument used to measure fluid viscosity and the flow properties of liquids.
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